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How to eliminate backlash in your drivetrain when running a mid drive motor.

How to eliminate backlash in your drivetrain when running a mid drive motor.

09-06-2018, 12:41 PM

Direct Drive works great to eliminate the backlash or noise from the slack in your drivetrain being taken up every time you hit the throttle.

I took my DNP freewheel, held it so the pawls are fully engaged and welded the backside so it can no longer freewheel. All of this was to eliminate backlash in the drivetrain...especially now that I am running a 50A controller with my BBSHD motor.

Background and more info below...
I broke a freewheel due to repeated impact loading. Everything I discuss is applicable to anything utilizing a ratchet/pawl type freewheel. Everytime I twisted the throttle, the slack in the drivetrain was taken up and a shock load was transmitted thru all of the drivetrain components. The courser (fewer engagement points) the engagement in the freewheel mechanism, the more slack there is and the higher the impact loads. Eventually things will start to break from the repeated shock loading on any bike.

After breaking a Shimano freewheel with 18 points of engagement (POE), I first tried a 30 POE DNP freewheel which helped some, then I tried a White Industries 72 POE freewheel, and then a Monty 135 POE freewheel. Each time the shock loading decreased but the problem was not eliminated. One issue I ran into when going to the 135 is that the largest they make it is with 18 teeth....normally used for bicycle trials. I planned to drill 4mm holes in six of the teeth and drill a sprocket with the same pattern and bolt them together. Great plan but the portion of the freewheel that has the ratcheting mechanism and the teeth is hardened to around Rockwell C60 (really hard) so when I tried to drill it with a High Speed Drill bit, the bit didn't even scratch the surface. If anyone attempts something similar, they need to use a Mill (to ensure perfect alignment of the bit and the surface being drilled) and a drill bit that is 100% carbide and made for drilling case hardened materials. They exist...a 4mm bit will run about $40 each.

My next step was to weld my DNP freewheel on the backside where the two pieces have a very small gap and load it so that the pawls were engaged while welding...that way the pawls still do all the driving and the weld just holds everything in place. Nice thing about the DNP freewheel is that it matches the ID of cassette gears and you can select any gear that you would run on a cassette and install it on the DNP freewheel....after you take it apart (which I am sure DNP never intended). The one way roller bearing inside the nylon gear of the BBSHD will still allow the rear wheel to overrun the motor. It is now a direct drive and the chain will move anytime the rear wheel moves...just like a motorcycle.

I am currently running a 30 tooth mighty mini front sprocket/chainring from Luna and three gears on the DNP cassette...an 18, 24, and 30. That gives me the 18 with a top speed of about 25 mph, the 24 for use in the woods with a top speed of about 18 mph, and the 30 if I want to ride around slowly and just explore and play. The derailleur is still there for shifting and chain tensioning.

What a great idea! I wonder if epoxy would work for those of us without welding equipment and skills? I have this problem on a freehub 10 speed. I'll have to take a look at that with this in mind. Onyx has some zero engagement hubs but they are $$$$$!

Comment

That is a cool mod, I forgot about this post. Good bump. Agree it should help durability. How's it held up,all good?

I ride a fat Ludicrous BBHSD. I just use the pedals to preload, usually. If going too fast or in wrong gear for that, I have throttle current start at 1%, so it's easy to be gentle. OK for close to 10k miles on my Sturgis Bullet, whatever rear hub that uses. But a friend broke his freewheel recently with a BBS02 on a MTB, so I sure recognize the risk.

BTW, we got him home by tying the granny gear to a few spokes, with safety wire from my kit. Field repair. Zip ties or string might even to it, in a pinch.